"20/20" "Stolen at birth" continues. Once again, Barbara Walters. It was the biggest kidnapping since the Lynn beindberg baby. Paul Fronczak, stolen at birth by a fake nurse. His angelic face peering from the front page was the last anyone had seen of him. Left behind were an empty crib and two broken hearts. A year passed. And then, one day, their phone rang. Eight hundred miles away in Newark, New Jersey, a boy had been found. The FBI contacted my parents and said, "We think we found your -- your son." Reporter: Whoever abandoned the baby, had dressed him up, wheeled him to a department store, in a new stroller, and walked away. 48 years later, we returned to the very place, with Paul Fronczak. Somebody wheels a stroller, puts you here, and walks away. And never looks back. And never looks back. It's crazy. Reporter: New Jersey authorities temporarily placed the abandoned boy with an unidentified family. That period was a mystery, until after we first aired this story. Then, Janet Ingrassia came forward. She contacted the "20/20" tip line with incredible new details. She says it was her family who had cared for the boy, as they had for nearly one hundred other foundlings over the years. He was an adorable little boy, he really was. The FBI is now investigating the little abandoned boy. They came several times, and took different, like, a mold of his ear, and that was sent up to Chicago. Reporter: Authorities compared the shape of the left ear of the boy in New Jersey with the baby photos of Paul Fronczak. One day, Janet's family was summoned to a meeting. They were told to bring the boy. He was led to a room where Dora and Chester Fronczak were waiting. And I heard the lady say, "Oh my god, this is my child. This is my baby." Everybody is such an, oh my god, the woman found her baby, and this baby found his mother. Reporter: The fronczaks took the little boy home with them to Chicago. And that was the last time Janet saw him. You have not seen Paul Fronczak in something like 50 years. That's right. Would you like to meet him now? Oh, I certainly would. Oh my god! Hello! Hi, how are you doing? How are you? I'm doing good, how are you? This is surreal, right? It is after so many years. It completes my heart. This brings back such good memories. Reporter: On a winter morning, Paul and Janet returned to the house in watching, New Jersey where he spent almost a year as a child. Paul remembers nothing of that time. Janet doesn't forget a thing. Your crib was -- your crib was over by the window, okay. You loved my father dearly. You really did, you know. You were the one that, uh, used to sit on his lap, you know. And, uh, you used to fall asleep on his lap and then he'd take you and put you inside. You were very happy here, Paul, you really were. Reporter: There was great happiness in the Fronczak home too. The family so often filmed in heartbreak, now recorded home movies of their own. The star of every celebration of course, was Paul. They had missed his first step, first word, first birthday. But Paul was now back where he belonged. Blissfully unaware of his past until years later. When you were ten years old, you had a shocking discovery, tell me about it. I was looking for Christmas presents and snooping around the house, and I found all these boxes, and it turned out it was a box of clippings and a bunch of cards and letters all about a kidnapping. Did you ask your parents about it? I did. I asked them, you know, what is this? And they said, well, you were kidnapped, we found you. Just like that, you were kidnapped, we found you? And that's all that matters, you're our son, we love you. How did your parents explain what happened to you? They really didn't talk about it, it was something that we really didn't bring up in the house. It was a very touchy subject. Did you ever feel that there was anything out of the ordinary when you were growing up? Well I, I did notice that I didn't resemble anybody in my family. You had a brother? Correct. And he looked like your parents? Exactly like my dad. Hundred percent. And you didn't at all? Not at all. Reporter: When he grew up, Paul pursued a career as an actor. His resemblance to George Clooney got him work as a stand-in in ocean's eleven movies. Ready, go. Today, Paul works for a college in Nevada where he lives with his wife Michelle and their daughter, Emma. Good job. Again, daddy. Do you remember how you felt when Paul said I'm not really sure who I am? The first time he told me I thought he was joking. I thought he was just kidding around. But then, once I realized it was true and I saw the newspaper clippings I felt very sad for his parents and sad for him. How do you think it has shaped him? I think as a 10-year-old boy, when he first saw those newspaper clippings, not realizing who he was, I'm sure that that has somehow shaped him and that's done something to him over the years. It's just sad. Was it sort of there in the back of your mind, this peculiar thing? It was really a big part of me through my whole life, and it's just gotten bigger and bigger and bigger. Who am I? Who am I? Yeah. Reporter: Then, last year Paul happened to see a DNA test kit for sale at a local drug store. At last, an easy answer to the question that had followed him all his life. The hard part, asking his parents for a DNA sample, forcing them to travel back in time, to the most painful part of their lives. It's something I wanted to do for a long time, but I never really had the nerve to -- to ask my parents. Reporter: Still ahead, will the Fronczak family agree to end decades of doubt? Paul is determined to find the truth, but his parents seem just as determined to keep it hidden. And they said, "Paul, please don't -- please don't send it

This transcript has been automatically generated and may not be 100% accurate.

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